The present invention relates to the reduction iron bearing material, such as iron ore, to metallic iron.
Many different iron ore reduction processes have been described and/or used in the past. The processes may be traditionally classified into direct reduction processes and smelting reduction processes. Generally, direct reduction processes convert iron ores into a solid state metallic form with, for example, use of shaft furnaces (e.g., natural gas-based shaft furnaces), whereas smelting reduction converts iron ores into molten hot metal without the use of blast furnaces.
The conventional reduction processes for production of direct reduced iron (DRI) involve heating beneficiated iron ores to below the melting point of iron, below 1200° C. (2372° F.), either by gas-based processes or coal-based processes. For example, in the gas-based process, direct reduction of iron oxide (e.g., iron ores or iron oxide pellets) employs the use of a reducing gas (e.g., reformed natural gas) to reduce the iron oxide and obtain DRI. Methods of making DRI have employed the use of materials that include carbon such as coal and coke as a reducing agent. A typical composition of DRI is 90 to 95% metallization and 2-4% gangue, but has shortcomings for steelmaking processes as a replacement of scrap because its oxygen and gangue content increases energy usage, increase slag volume, and necessitates the addition of costly reagents.
Natural gas-based direct reduced iron accounts for over 90% of the world's production of DRI. Coal-based processes are generally used in producing the remaining DRI production. However, in many geographical regions, the use of coal may be more desirable because coal prices may be more stable than natural gas prices. Further, many geographical regions are far away from steel mills that use the processed product. Therefore, shipment of iron units in the form of iron nuggets produced by a coal-based direct reduction process may be more desirable than use of a smelting reduction process.
Another reduction process in gas-based or coal-based directly reducing iron bearing material to metallic nuggets is often referred to as fusion reduction. Such fusion reduction processes, for example, generally involve the following processing steps: feed preparation, drying, preheating, reduction, fusion/melting, cooling, product discharge, and metallic iron/slag product separation. These processes result in direct reduction of iron bearing material to metallic iron nuggets and slag. Metallic iron nuggets produced by these direct reduction processes are characterized by high grade reduction, nearing 100% metal (e.g., about 96% to about 97% metallic Fe). Percents (%) herein are percents by weight unless otherwise stated.
Unlike conventional direct reduced iron (DRI), these metallic iron nuggets have low oxygen content because they are metallic iron and have little or no porosity. These metallic iron nuggets are also low in gangue because silicon dioxide has been removed as slag. Such metallic iron nuggets are desirable in many circumstances such as use in place of scrap in electric arc furnaces. These metallic iron nuggets can be also produced from beneficiated taconite iron ore, which may contain 30% oxygen and 5% gangue. As a result, with such metallic iron nuggets, there is less weight to transport than with beneficiated taconite pellets and DRI, as well as a lower rate of oxidation and a lower porosity than DRI. In addition, generally, such metallic iron nuggets are just as easy to handle as taconite pellets and DRI.
Various types of hearth furnaces have been described and used for direct reduction of metallic iron nuggets. One type of hearth furnace, referred to as a rotary hearth furnace (RHF), has been used as a furnace for coal-based direct reduction. Typically, the rotary hearth furnace has an annular hearth partitioned into a preheating zone, a reduction zone, a fusion zone, and a cooling zone, between the supply location and the discharge location of the furnace. The annular hearth is supported in the furnace to move rotationally. In operation, raw reducible material comprising a mixture of iron ore and reducing material is charged onto the annular hearth and provided to the preheat zone. After preheating, through rotation, the iron ore mixture on the hearth is moved to the reduction zone where the iron ore is reduced in the presence of the reducing material and fused into metallic iron nuggets, using one or more heat sources (e.g., gas burners). The reduced and fused product, after completion of the reduction process, is cooled in the cooling zone on the rotating hearth, preventing oxidation and facilitating discharge from the furnace.
One exemplary metallic iron nugget direct reduction process for producing metallic iron nuggets is referred to as ITmk3® by Kobe Steel. In such a process, dried balls formed using iron ore, coal, and a binder are fed to a rotary hearth furnace. As the temperature increases in the furnace, the iron ore concentrate is reduced and fuses when the temperature reaches between 1450° C. to 1500° C. The resulting products are cooled and then discharged. The intermediate products generally are shell-shaped, pellet-sized metallic iron nuggets with slag inside, from which the metallic iron can be separated.
Another direct reduction process for making metallic iron nuggets has also been reportedly used. See U.S. Pat. No. 6,126,718. In this process, a pulverized anthracite coal layer is spread over a hearth and a regular pattern of dimples is made therein. Then, a layer of a mixture of iron ore and coal is placed over the dimples, and heated to 1500° C. The iron ore is reduced to metallic iron, fused, and collected in the dimples as iron pebbles and slag. Then, the iron pebbles and slag are broken apart and separated.
Both of these direct reduction processes for producing metallic iron nuggets have involved mixing of iron-bearing materials and a carbonaceous reductant (e.g., pulverized coal). Either with or without first forming dried balls, iron ore/carbon mixture is fed to a hearth furnace (e.g., a rotary hearth furnace) and heated to a reported temperature of 1450° C. to approximately 1500° C., to form metallic iron nuggets and slag. Metallic iron and slag can then be separated, for example, with use of mild mechanical action and magnetic separation techniques.
A particular problem with the metallic iron nuggets formed by these previous direct reduction processes was the sulfur content of the nuggets. Sulfur is a major impurity in direct reduced metallic iron nuggets. In the past, carbonaceous reductants utilized in direct reduction processes of iron ore have generally resulted in metallic iron nuggets with at least 0.1% or more by weight sulfur. This high level of sulfur has made the metallic iron nuggets made by direct reduction undesirable in many steelmaking processes, and particularly in the electric arc furnace processes.
Attempts have been made to form metallic iron nuggets with low sulfur content in these previous direct reduction processes using large amounts of additives containing MgCO3 or MgO. Problems, such as increased energy consumption and increased refractory wear, have occurred with fusing these nuggets due to the increases in slag melting temperature caused by MgO in the slag. See EP 1 605 067.
A method and system are disclosed that provide for various advantages in the reduction processes in the production of metallic iron nuggets. The method and system results in a marked higher percent of the sulfur in the slag without the use of large amounts of Mg compounds, and a marked lower percent of the sulfur in the metallic iron nuggets. A novel intermediate metallic nugget/slag product having a ratio of percent weight sulfur in the slag to sulfur in the metallic nugget of at least 12, at least 15 or at least 30, without large amounts of MgO within the slag, which may result in nuggets with less than 0.05% sulfur content. A novel metallic nugget having a sulfur content less than 0.03% by weight may also be produced by the disclosed process.
A method for use in production of metallic iron nuggets is disclosed that comprises providing a hearth refractory material, providing a hearth material layer comprising at least carbonaceous material on the refractory material, providing a layer of reducible mixture comprised of at least reducing material and reducible iron bearing material arranged in a plurality of discrete portions over at least a portion of the hearth material layer, providing a layer of coarse carbonaceous material over at least a portion of the discrete portions of reducible mixture, and heating the reducible mixture to form one or more discrete portions into an intermediate product of metallic iron nuggets and slag, and after separation, metallic iron nuggets. The step of heating the reducible mixture may form singular metallic iron nuggets with separate slag portions from a majority of the discrete portions. The overlayer is generally provided prior to heating, but may be provided after devolatilization of carbonaceous material occurs and before completion of solid state reduction.
The coarse carbonaceous material of the overlayer has an average particle size greater than an average particle size of the hearth layer. In addition or alternatively, the overlayer of coarse carbonaceous material may include discrete particles having a size greater than about 20 mesh or greater than about 6 mesh, and in some embodiments, the overlayer of coarse carbonaceous material may have discrete particles with a size between about 20 mesh or about 6 mesh and about ½ inch (12.7 mm). The coarse carbonaceous material may be coke, non-caking coal, char, or a combination of one or more of these. In the alternative, the overlayer of coarse carbonaceous material may have discrete particles with a size between about ⅜ inch (9.7 mm) and about ½ inch (12.7 mm) or between about 3 mesh (6.7 mm) and about ⅜ inch (9.7 mm).
In addition or in the alternative, the discrete particles of the hearth layer may have a particle size less than 4 mesh, and in some embodiments a particle size between 100 and 20 mesh or 6 mesh. Particle sizes less than 100 mesh should be avoided because these particles sizes tend to have more ash content. The thickness and particle size of the carbonaceous and other material in the hearth layer should be selected so that the hearth layer protects the hearth refractory from slag and molten metal formed during reduction of the reducible mixture, while avoiding production of excess ash. The hearth layer may have a particle size between a range of −6 to −20 mesh to a range of +65 to +150 mesh. The carbonaceous material in the reducible mixture is also different in particle size from those of the coarse overlayer, but for the different considerations. In the reducible mixture a consideration is the surface area for rapid reaction of the carbonaceous material with the reducible iron bearing material in commercial production. Less than 65 mesh or less than 100 mesh particle size of carbonaceous material in the reducible mixture is effective for efficient reduction of the iron oxide to produce metallic iron nuggets.
The overlayer of coarse carbonaceous material may provide between 50% and 100% coverage of the discrete portions of reducible mixture and may be about ½ inch (12.7 mm) in thickness. Further, in some embodiments of the method, the coverage of the overlayer of coarse carbonaceous material may be between about 0.5 lb/ft2 (2.44 kg/m2) and about 1 lb/ft2 (4.88 kg/m2) of coarse carbonaceous material, or between about 0.75 lb/ft2 (3.66 kg/m2) and about 1 lb/ft2 (4.88 kg/m2) of coarse carbonaceous material over the reducible mixture.
In some embodiments of the disclosed method, the step of providing a reducible mixture over at least a portion of the hearth material layer may comprise forming at least a portion of the reducible mixture with a predetermined quantity of reducing material between about 70 percent and about 90 percent of said stoichiometric amount of reducing material necessary for complete metallization. Said stoichiometric amount may be between about 75 percent and about 85 percent of said stoichiometric amount of reducing material for complete metallization, or about 80 percent of said stoichiometric amount of reducing material for complete metallization. The stoichiometric amount of reducing material is the calculated amount of carbonaceous material needed for complete metallization of iron in the formation of metallic iron nuggets from a predetermined quantity of reducible iron bearing material.
The discrete portions may be formed in situ as mounds, or alternatively, preformed as briquettes, balls, extrudates, or other shapes as needed. In any event, the discrete portions of reducible mixture may be at least partially surrounded with nugget separation fill material comprising at least carbonaceous material. The fill material may be placed by depositing the carbonaceous material after the discrete portions are formed, or by dropping or pushing preformed discrete portions into the hearth layer. The nugget separation fill material may also have an average particle size less than the average particle size of the coarse carbonaceous material of the overlayer. However, the step of providing an overlayer of coarse carbonaceous material may comprise at least partially surrounding the discrete portions of reducible mixture with coarse carbonaceous material. In some embodiments, this may be accomplished by placing the coarse carbonaceous material over the discrete portions of reducible mixture and allowing some of the coarse carbonaceous material to go between the discrete portions of reducible mixture.
In some embodiments of the method, the step of heating the layer of reducible mixture includes heating the layer of reducible mixture at a temperature of less than about 1425° C. Also, the step of thermally heating the layer of reducible mixture may include heating the layer of reducible mixture at a temperature of less than about 1400° C. or less than 1375° C.
Also disclosed is a method for use in production of metallic iron nuggets that comprises providing a hearth refractory material, providing a hearth material layer comprising at least carbonaceous material on the refractory material, providing a layer of reducible mixture comprised of at least reducing material and reducible iron bearing material arranged in a plurality of discrete portions over at least a portion of the hearth material layer, providing a layer of turbulent gas flow disrupting material over at least a portion of the discrete portions of reducible mixture, and heating the reducible mixture to form one or more discrete portions into an intermediate product of metallic iron nuggets and slag, and after separation, metallic iron nuggets. In this alternative method, at least partially surrounding the discrete portions of reducible mixture may be nugget separation fill material comprising at least carbonaceous material. Also, in some embodiments, the step of providing an overlayer of turbulent gas flow disrupting material may include providing coarse carbonaceous material. Further, the overlayer of turbulent gas flow disrupting material may include providing between about 0.5 lb/ft2 (2.44 kg/m2) and about 1 lb/ft2 (4.88 kg/m2) of coarse carbonaceous material, or between about 0.75 lb/ft2 (3.66 kg/m2) and about 1 lb/ft2 (4.88 kg/m2) of coarse carbonaceous material.
Also disclosed is an intermediate product comprising metallic iron nuggets and slag having less than 5% MgO and having a ratio of percent by weight sulfur in the slag to sulfur in the metallic nuggets of at least 12, at least 15, or at least 30, which may produce nuggets of less than 0.05% sulfur. In addition, a metallic iron nugget composition having a sulfur content less than 0.03% by weight is disclosed. The metallic nugget/slag product and metallic iron nuggets may be produced by the method steps that comprise providing a hearth refractory material, providing a hearth material layer comprised of at least carbonaceous material on the refractory material, providing a layer of reducible mixture comprising at least reducing material and reducible iron bearing material arranged in a plurality of discrete portions over at least a portion of the hearth material layer, optionally at least partially surrounding the discrete portions of reducible mixture with nugget separation fill material comprising at least carbonaceous material, providing a layer of coarse carbonaceous material over at least a portion of the discrete portions of reducible mixture, and heating the reducible mixture to form the one or more discrete portions into the intermediate product of metallic iron nuggets and slag of said sulfur slag/nugget ratio, and after separation, metallic iron nuggets. The slag formed may have an iron content of less than about 1%, less than about 0.25%, or essentially less than 0.1%.
The carbonaceous material of the hearth layer, the coarse overlayer, and the layer of reducible mixture may contain an amount of sulfur in a range from about 0.2% to about 1.5%, and more typically, in the range of 0.5% to 0.8%. The reducible mixture may also contain an amount of additives in a range from about 1% to about 10%. The reducible mixture may further include an additive selected from the group consisting of SiO2, CaF2, Na2CO3, aluminum smelter slag, cryolite, fluorspar and soda ash. The additives may be separately added to the reducible mixture in its making, or may be naturally part of the reducible iron bearing material and/or the carbonaceous material used in making the reducible mixture. Typically 2% of the content of the reducible mixture may be additives, but may range between about 1% and about 7% by weight. Compounds containing Mg, such as dolomite, should be avoided, and in any event compounds containing Mg are not added in quantities such that greater than 3%, or greater than 4%, or greater than 5% MgO results in the slag.
The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings.